Now that we’ve explored how to collect user input using fgets(), let’s take another step forward by allowing users to pass input directly when running the program using command-line arguments.

In C, command-line arguments are handled using the main() function’s parameters: int argc and char *argv[]. Where argc is the argument count and *argv[] an array of strings representing the arguments.

Here’s a simple example:

#include <stdio.h>

int main(int argc, char *argv[]) {
    // Check if a name was provided as an argument
    if (argc > 1) {
        printf("Hello, %s!\n", argv[1]);
    } else {
        printf("Hello, World!\n");
    }

    // Print the number of arguments passed (including the program name)
    printf("Number of arguments: %d\n", argc);

    // Print all arguments
    for (int i = 0; i < argc; i++) {
        printf("Argument %d: %s\n", i, argv[i]);
    }

    return 0;
}

Breaking Down the Code

1. The main() Parameters

int main(int argc, char *argv[])

• argc: Short for argument count. It tells you how many command-line arguments were passed.
• argv[]: Short for argument vector. It’s an array of strings (character pointers), each holding one of the arguments, including the program name.

2. Conditional Greeting

if (argc > 1) {
    printf("Hello, %s!\n", argv[1]);
} else {
    printf("Hello, World!\n");
}

• If the user provides a name as the first argument (argv[1]), the program prints a personalized greeting.
• If no name is provided, it defaults to "Hello, World!".

3. Looping Over Arguments

for (int i = 0; i < argc; i++) {
    printf("Argument %d: %s\n", i, argv[i]);
}

This loop prints all the arguments passed to the program, showing how data flows into C programs via the command line.

Example Output

When run like this:

./greet Laura Linux

Output:

Hello, Alice!
Number of arguments: 3
Argument 0: ./greet
Argument 1: Laura
Argument 2: Linux

When run with no arguments:

./greet

Output:

Hello, World!
Number of arguments: 1
Argument 0: ./greet

Summary

This version of the program demonstrates how to:

• Accept input from the command line using argc and argv.
• Use conditionals to modify behavior based on user input.
• Loop through arguments to process dynamic data.

Command-line arguments are a powerful way to make your C programs flexible and script-friendly, especially in automation and scripting tasks.